As diesel engine emissions legislation becomes more stringent, a number of new technologies are under investigation and fall in the general category of “after-treatment”. These technologies include, but are not limited to diesel particulate filters, oxidation catalysts, and NOx traps. Most after-treatment filters, catalysts, traps, etc., which may be referred to as after-treatment units, require some sort of “regeneration” to refresh their emissions reducing capacity.
Regeneration techniques vary from technology to technology, but usually involve changing either the temperature or equivalence ratio (e.g., air to fuel ratio relative to a stoichiometric ratio) of the exhaust. For example, a diesel particulate filter typically requires quite high temperatures to burn off particulates trapped in the filter. As another example consider the NOx trap, which typically requires regeneration several times per minute. During regeneration of a NOx after-treatment unit, the air to fuel ratio, which normally runs lean typically at approximately 19:1 to approximately 27:1 at full load and much higher at part load, is reduced to achieve rich combustion (e.g., an air to fuel ratio at or below approximately 14:1). However, various problems may be encountered when operating at such low air to fuel ratios. For example, depending on combustion temperature, unsatisfactory level of smoke may be generated at low air to fuel ratios.
Traditionally, an after-treatment unit is placed in an engine exhaust stream after an exhaust turbine. During regeneration, the engine is operated in a significantly different thermodynamic regime than during normal operation. The thermodynamic regime suited to regeneration may have a substantial impact on engine operation. For example, such a thermodynamic regime may confound control of torque to maintain a commanded level by an operator or control of an air management system that includes a turbocharger (e.g., to maintain a smooth airflow). In particular, during a typical 2 to 4 second NOx unit regeneration, a reduction in mass flow occurs across the entire engine, typically by a factor of approximately two. Such a reduction in mass flow results in unsatisfactory conditions for turbocharger operation. For example, during regeneration, a significant variation in turbine speed may occur, which may cause undesirable pressure gradients at the inlet manifold that can result in further outlet manifold pressure disturbances.
Overall, after-treatment regeneration presents tremendous challenges in engine management control and system design where acceptable emissions and operator satisfaction are imperative. Hence, a need exists for new or improved methods, devices and/or systems for after-treatment regeneration. Various exemplary methods, devices and/or systems presented below meet this need and/or other needs.